1. Technical Field
Optical circuitry, as included in all-optical as well as electro-optical integrated circuits, is characterized by high population density as well as efficient operation of optical elements-active and/or passive.
2. Description of the Prior Art
Dominance of optical transmission for use in most high capacity long distance systems is now well established. Concentrated effort both here and abroad has resulted in expedient manufacture, installation, and use of fiber which satisfies the motivating promise of high capacity, low loss transmission. While not as well developed, the status of medium and short distance is not very different. Active backplane as well as microstrip media are on their way to satisfying needs.
For the most part advances in optical transmission have not been matched in interfacing circuitry. Most operating systems rely on electronics for interfacing with optical transmission. Modulation of the optical carrier, generation of optical signals to be inserted in the transmission line, detection of such signals at line terminals, amplification at repeater stations, etc. all depend upon electronics rather than photonics. Equipment expense as well as operating problems at electronic-optical interfaces have provoked worldwide effort toward development of all-optical equipment.
While interfacing with optical transmission system is properly regarded as the chief motivation, at least on the shorter term, inherent properties of optics provoke effort directed toward optical circuitry. The ultimate ambition is all-optical in terms of device function. At this time, and probably for some time in the future, convenience of electronic biasing/pumping suggests that circuitry will take the form of opto-electronic integrated circuits. The likelihood that electronic elements will continue to be of choice for some functions supports this conclusion.
Circuit density has been a major concern in the development of optical ICs--as regards both transmission paths and devices, e.g. amplifiers, switches, etc. Unwanted evanescent field coupling--a phenomenon of little consequence in electronics--imposes spacing limitations as between adjacent paths and devices. Effort directed toward optical IC chips to perform all but the simplest functions has been thwarted by crosstalk and other consequences of unwanted coupling. Lessening of such effects has led to spacings generally of tens of micrometers--generally to tens of wavelengths as measured in vacuum. (The two spacing measures are quite similar for popular operating wavelengths from .apprxeq.0.9 to 1.55 .mu.m as available from popular GaAs and InP-based devices.) Use of design rules of .apprxeq.10 .mu.m would satisfy some short term conservative objectives, but even at that level, unwanted coupling, likely above .apprxeq.10%, is objectionable both in terms of noise and of energy/heat dissipation.